1. Field of the Invention
The present invention relates to a method for manufacturing a toner for use in development of an electrostatic latent image in image formation by an electrophotographic method.
2. Description of the Related Art
In the electrophotographic method, an image is formed as follows. A surface of an electrophotographic photoreceptor (hereinafter also referred to as merely a “photoreceptor”) is charged, and then exposed to light to form an electrostatic latent image, and a toner is electrostatically attracted onto the surface of the photoreceptor to develop the electrostatic latent image, and a toner image obtained is transferred from the surface of the photoreceptor to a recording medium such as a sheet of paper and fixed. The toner is charged by a frictional charge, transported by a developer bearing member such as a developing roller, and supplied onto the surface of the photoreceptor. The toner that remains on the surface of the photoreceptor without being transferred onto the recording medium is physically scraped by a removing section to be removed from the surface of the photoreceptor.
As the removing section of a toner, a cleaning blade is used due to its simple configuration and its good removing capability. The cleaning blade removes a toner by sliding and scrubbing on the surface of the photoreceptor to scrape toner particles constituting the toner. At this time, the more a shape of the toner particles is similar to a true spherical shape, the less rolling friction force acting on the toner particles from the photoreceptor and the cleaning blade is. Therefore, so-called poor cleaning, in which the toner particles roll in a space between the photoreceptor and the cleaning blade to slip through the cleaning blade, and remain on the surface of the photoreceptor without being scraped by the cleaning blade, has a tendency to occur. In addition, the smaller a particle diameter of the toner particles becomes, the more easily the toner particles enter the space between the photoreceptor and the cleaning blade. Accordingly, the smaller the particle diameter of the toner particles becomes to obtain high image quality, the more easily the poor cleaning has a tendency to occur. Therefore, in order to prevent occurrence of the poor cleaning and to improve a cleaning capability of a toner, the toner particles preferably have a shape (hereinafter also referred to as an “irregular shape”) out of the true spherical shape.
As described above, it is preferable that the toner particles have a shape out of the true spherical shape in terms of the cleaning capability. However, the more the toner particles have a shape similar to the true spherical shape, the more preferable in terms of a flow property. When the toner particles have a shape significantly out of the true spherical shape, the flow property of the toner is decreased to cause an uneven charge amount, compared with a case in which the toner particles have a shape similar to the true spherical shape. Accordingly, scattering of the toner and fogging on an image because of the scattering may occur. In addition, the toner may not be stably supplied onto the surface of the photoreceptor, thus preventing development thereof. Therefore, in order to achieve a toner balancing between the cleaning capability and the flow property, it is preferable that the toner particles have a shape (hereinafter referred to as an “approximately spherical shape”) slightly out of the true spherical shape.
Examples of a method for manufacturing a toner include a so-called pulverization method, in which for example a toner composition containing at least a colorant and a binder resin is kneaded to produce a kneaded material, and then the kneaded material obtained is solidified and directly pulverized to obtain toner particles (refer to Japanese Unexamined Patent Publications JP-A 5-249735 (1993), p. 11, and JP-A 8-234480 (1996), p. 15, for example). In the pulverization method, the kneaded material is solidified and directly pulverized to obtain toner particles. Accordingly, it is difficult to control a toner particle shape, and it is thus difficult to manufacture toner particles having the approximately spherical shape.
Examples of a method for manufacturing a toner other than the pulverization method include polymerization methods such as a suspension polymerization method and an emulsion polymerization method. In the polymerization method, a binder resin monomer containing a colorant is dispersed into water, and then polymerized to obtain toner particles. As described above, in the polymerization method, toner particles are produced in water, resulting that the toner particles have a tendency to have a shape similar to the true spherical shape that is stable in water. In the polymerization method, examples of a method for producing toner particles having the irregular shape include, a method in which a monomer capable of cross-linking is used as the binder resin monomer, and the monomer is polymerized in water to produce toner mother particles having a shape similar to the true spherical shape, and the toner mother particles are subjected to a cross-linking reaction to partially cross-link the binder resin, thereby obtaining the toner particles having the irregular shape; and a method in which toner particles having a shape similar to the true spherical shape are once produced in water, and the toner particles obtained are subjected to shear force to obtain the toner particles having the irregular shape (refer to Japanese Unexamined Patent Publication JP-A 2005-173578, for example).
In the method for producing toner particles having the irregular shape by the cross-linking reaction, a shape of the toner particles is changed depending on a degree of cross-linking, though it is difficult to control the degree of cross-linking. Accordingly, it is difficult to manufacture toner particles having the approximately spherical shape. In addition, in the method for producing toner particles having the irregular shape by shear force, there has been a problem in which it is necessary to create a step for producing the toner particles having the irregular shape after producing the toner particles by conventional polymerization methods or the like, thus making manufacturing steps complicated. Moreover, the degree of cross-linking depends on the shear force, though it is difficult to finely control the shear force, and it is thus difficult to manufacture toner particles having the approximately spherical shape.
Furthermore, as another method for producing toner particles having the irregular shape, there is an emulsion flocculation method (refer to Japanese Examined Patent Publication JP-B2 3435586, p. 5, for example). In the emulsion flocculation method, a binder resin monomer is emulsified and polymerized in water to produce resin particles having a diameter smaller than that of objective toner particles, and then a water dispersion of the resin particles obtained is mixed with a water dispersion of colorant particles or the like to flocculate the resin particles together with the colorant particles, to produce botryoidal toner particles. In the emulsion flocculation method, there has been a problem in which it is necessary to have a step for flocculating the resin particles after once producing the resin particles, thus making manufacturing steps complicated. In addition, it is extremely difficult to control a degree of flocculation of the resin particles and the colorant particles in order to control a shape of the toner particles.
As described above, in any method of the pulverization method, the polymerization method, and the emulsion flocculation method, it is difficult to control a shape of toner particles, and it is thus difficult to manufacture toner particles having the approximately spherical shape.